  //返回顶部
  const backtop = document.querySelector(".backtop");
  onscroll = function () {
    var scrollTop = Math.floor(document.documentElement.scrollTop);
    // console.log(scrollTop);
    if (scrollTop >= 480) {
      backtop.style.display = "block";
      backtop.style.opacity = "1";
    } else {
      backtop.style.display = "none";
      backtop.style.opacity = "0";
    }
  };
  function clickit() {
    document.documentElement.scrollTop = 0;
  }
  backtop.onclick = function () {
    scrollTo({
      top: 0,
      behavior: "smooth",
    });
  };